1. Field of the Invention
The present invention relates to an electrically conducting composition. More particularly, the present invention relates to an electrically conducting composition that includes a charge transporting oligomer for organic electronic devices.
2. Description of Related Art
There has been an increasing demand in developing novel organic materials that cater to organic light emitting device (OLED) applications. Such devices are commercially attractive because they offer the cost-effective fabrication of high density pixeled displays exhibiting brilliant luminance with long life times, high efficiency, low driving voltages and wide color range.
A typical OLED has at least one organic emissive layer sandwiched between an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton”, which is a localized electron-hole pair having an excited energy state, is formed. Light is emitted when the exciton relaxes through a photo emissive mechanism. To improve the charge transport capabilities and also the luminous efficiency of such devices, additional layers around the emissive layer, such as an electron transport layer and/or a hole transport layer, or an electron blocking and/or hole blocking layer(s) have been incorporated. Doping the host material with another material (guest) has been well demonstrated in literature to enhance the device performance and to tune the chromaticity. Several OLED materials and device configurations are described in U.S. Pat. Nos. 4,769,292, 5,844,363, and 5,707,745, which are incorporated herein by reference in their entirety.
Indium-tin oxide (ITO) possesses a relatively low work function (WF=4.9 eV), new hole-injection layers (HILs) with a high ionization potential (IP) are required for efficient hole injection. Small molecules such as such as copper phthalocyanine (CuPc); 4,4′,4″-tris(3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA); and 4,4′,4″-tris[N-(2-naphthyl)-N-phenyl amino]triphenylamine (2TNATA), have been preferentially employed as HILs in vacuum-deposited, small-molecule organic light-emitting diodes (OLEDs). Although conducting polymers such as poly (ethylene dioxythiophene) doped with polystyrene sulfonic acid (PEDOT:PSS) and polyaniline doped with polystyrene sulfonic acid (PANI:PSS) are alternatives to small-molecule HILs in OLEDs, only a few reports have described solution-processed HILs in OLEDs because of the remaining high injection barrier and low efficiency.
Polyaniline and polythiophenes, which are widely used hole injection material in polymeric light emitting devices (PLED), exhibits a device deterioration due to the residual water content and are difficult to control purity as described in JP 2009069523, JP 2009069523.
Patent documents WO2005107335, WO2004043117 have described conductive coating materials made of homogeneos solutions containing low-molecular weight oligoaniline or oligothiophene materials are useful substitutes for conventional materials. The low molecular weight oligomeric compounds of the charge transporting material significantly changes the physical properties, such as charge transporting performance, ionization potential, conductivity, solubility, viscosity, film morphology, etc. For example, the oligoaniline derivative has the quinoimine in its oxidized state, which remarkably reduces the solubility in organic solvents but plays a main role to its conductivity. By means of a chemical reduction of the quinoimines with hydrazine, the solubility of the oligoaniline may be improved partially; however, the conductivity of the film is very low. Doping of oligoanilines with conductivity enhancing salts without decreasing the solubility, remains a challenge in the solution processible OLEDs.